Andalusite is an alumnosilicate that forms under low temperature and pressure. It has strong trichroism and often displays different colours in green/brown/pink. Gem material is known from Brazil, Burma, Russia, Sri Lanka, USA and Madagascar. Andalusite is generally not treated, and no synthetic andalusite is known reported.
Apatite is a rather soft phosphate mineral that comes in many different colours. A well appreciated variety of apatite is blue (similar to Paraiba tourmaline in colour) which is a heat-treated variety of apatite. Apatite is commonly found in Brazil, Burma, Madagascar, Sri Lanka. Cat’s eye apatite in different colours can also be found in the trade.
Diaspore is an aluminium oxide hydroxide rarely found in gem-quality. Diaspore is known for its distinct colour change. Stones from mines in Southwestern Turkey are marketed as Zultanite or Csarite.
Diopside is usually yellowish green to dark brown in colour that forms in metamorphic environments. Chrome diopside (mostly from Russia) is a vivid green variety. Other colours are also possible and diopside may show nice star effects.
Feldspars are the most abundant silicates minerals found in the earth’s crust. They are the main constituent of many different igneous, volcanic, metamorphic and sedimentary rocks.
Feldspars with significance as gemstones are mainly from pegmatites (e.g. amazonite, orthoclase), plutonites (e.g. spectrolite), vulcanite (e.g. labradorite), and rarely alpine clefts(e.g. adularia). The most common varieties used for jewellery are amazonite, sanidine, labradorite, moonstone, andesine and sunstone.
Fluorite is a soft and easily cleavable mineral that comes in many different colours. Fluorites can be colour zoned. Due to the perfect cleavage, fluorite is seldom used in jewellery.
Lapis lazuli is an opaque rock consisting of different minerals appreciated for its deep blue colour. Lapis often has a mottled and slightly inhomogeneous texture. It has been used in jewellery for millennia and is often cut in cabochons or other ornamental shapes. Lapis-lazuli from the Sar-E-Sang mines in Afghanistan has been traded and used in Central Asia for at least 4000 years (Wyart et al., 1981). This continues to be the best source of lapis-lazuli worldwide at present. Lapis lazuli is usually not treated.
Malachite is a green copper mineral that has been popular since ancient times. It is opaque and known for its banding texture. Commercially important mines are found in DRC and Russia. Malachite dust is potentially toxic and thus great care must be taken during the cutting and polishing process.
Opal is an amorphous silica famous for its beautiful play of colour (opalescence). Varieties include boulder opal, white opal, black opal and fire opal. Common opal (from Peru for example) is opal showing no play of colour and can be found in blue and pink colours mainly. Commercial deposits are found in Australia, Brazil, Ethiopia, Mexico and Peru. Opal can form in sedimentary or volcanic deposits.
Gem scapolite forms in metamorphic rocks and can be found in Burma, Madagascar and Tanzania. Colours range from colourless to yellow, brown and purple. Scapolite can be treated to improve colour.
Spodumene is a lithium rich pyroxene mineral. Gem varieties range in colour from colourless to yellow, purplish and green. Spodumenes are found in pegmatite deposits. Main gem deposits today include Afghanistan, Brazil, Madagascar, Pakistan and USA. Kunzite is the pink variety of spodumene, and hiddenite the green variety. Spodumenes may be treated to improve colour, for example through irradiation. They are often very pure stones and can cleave easily.
Turquoise is blue to green in colour and has been used for millennia in jewellery and in ornamental objects. It is often found as a secondary mineral in and near cooper deposits. Turquoise is frequently treated (through stabilisation) and both imitations and synthetic turquoise exist. Found since ancient times in Iran, other mines are found in the US, Chile and China.
Morganites are the pink-orange variety of beryl. They can be found in Afghanistan, Brazil, Nigeria, Mozambique, Madagascar, Pakistan. They are found in pegmatites. They can commonly be treated through heat treatment.